Electrode catalyst, composition for forming gas diffusion electrode, gas diffusion electrode, membrane-electrode assembly, and fuel cell stack

ABSTRACT

Provided is an electrode catalyst that can exhibit sufficient performance, is suitable for mass production, and is suitable for reducing production costs, even when containing a relatively high concentration of chlorine. The electrode catalyst has a core-shell structure including a support; a core part that is formed on the support; and a shell part that is formed so as to cover at least one portion of the surface of the core part. A concentration of bromine (Br) species of the electrode catalyst as measured by X-ray fluorescence (XRF) spectroscopy is 500 ppm or less, and a concentration of chlorine (Cl) species of the electrode catalyst as measured by X-ray fluorescence (XRF) spectroscopy is 8,500 ppm or less.

TECHNICAL FIELD

The present invention relates to an electrode catalyst. Also, thepresent invention relates to a composition for forming a gas diffusionelectrode including the electrode catalyst, a gas diffusion electrode, amembrane-electrode assembly, and a fuel cell stack.

BACKGROUND ART

A so-called polymer electrolyte fuel cell (Polymer Electrolyte FuelCell: hereinafter called “PEFC” as needed), has its operatingtemperature of from a room temperature to about 80° C. Also, since PEFCmakes it possible to employ inexpensive general-purpose plastics, etc.for members constituting its fuel cell body, it is possible to realizereduction in weight. Furthermore, PEFC makes it possible to achievethinning of a polymer electrolyte membrane, enabling an electricresistance to be reduced, thereby enabling a power loss to be reducedrelatively easily. Due to PEFC having not a few advantages as describedabove, it is applicable to a fuel cell vehicle, a home cogenerationsystem, and the like.

As an electrode catalyst for PEFC, there has been proposed an electrodecatalyst in which a platinum (Pt) or platinum (Pt) alloy, i.e., acomponent for the electrode catalyst, is supported on a carbon servingas a support (for example, Patent Document 1, Non-Patent Document 1).

Conventionally, there have been disclosed that, as for an electrodecatalyst for PEFC, if the content of chlorine contained in the electrodecatalyst is 100 ppm or more, it is not desirable as an electrodecatalyst (for example, Patent Document 2); and that this is because ifthe content of chlorine contained in the electrode catalyst is 100 ppmor more, it is impossible to obtain a sufficient catalytic activity forthe electrode catalyst for fuel cells; and corrosion of its catalystlayer will occur, thus shortening the life of the fuel cell.

Then, there is disclosed, as the catalyst component of the electrodecatalyst, a powder of platinum (Pt) or platinum (Pt) alloy that containsless than 100 ppm of chlorine (for example, Patent Document 2).

As for the preparation of a powder of the platinum (Pt) or platinum (Pt)alloy, there is disclosed the following method: forming a melt whichcontains a low-melting mixture of alkali-metal nitrate, a chlorine-freeplatinum compound and a chlorine-free compound of alloying elements;heating the melt up to a reaction temperature at which the platinumcompound and the compound of the alloying elements are thermallydecomposed to give an oxide; cooling the melt; and the melt is dissolvedin water and the resulting oxide or mixed oxides are converted into apowder of platinum or platinum alloy by successive reduction.

Incidentally, the present applicant submits, as publications where theabove-mentioned publicly-known inventions are described, the followingpublications:

PRIOR ART DOCUMENT Patent Document

-   Patent Document 1: Japanese Un-examined Patent Application    Publication No. 2011-3492-   Patent Document 2: Japanese Un-examined Patent Application    Publication No. 2003-129102 (Japanese Patent No. 4,286,499)

Non-Patent Document

-   Non-Patent Document 1: MATSUOKA et al., “Degradation of Polymer    Electrolyte fuel cells under the existence of anion species”, J.    Power Sources, 2008, May 1, Vol. 179 No. 2, P. 560-565

SUMMARY OF THE INVENTION Problem to be Solved by the Invention

As mentioned above, from the viewpoint of improving the catalyticactivity and lifetime of PEFC as the electrode catalyst, it is importantto reduce the content of chlorine contained in the catalyst.

However, from the viewpoint of seeking to simplify the manufacturingprocess and reduce the manufacturing cost for the practical use of PEFC,there has been room for improvement in the conventional arts describedabove.

That is, according to the aforementioned electrode catalyst having achlorine content of less than 100 ppm, there has been a need to preparethe same through a complex process for removing chlorine as disclosed inPatent Document 2, etc., and hence there has been room for improvement.

Thus, when assuming a future mass production of PEFC, it is consideredthat there will be required an electrode catalyst that can demonstrate asufficient performance even when having a relatively high chlorineconcentration as high as more than 100 ppm, and can be prepared withouta special and complicated process for eliminating chlorine such that theelectrode catalyst is suitable for mass production and reducing themanufacturing cost.

The present invention has been made in view of such technicalcircumstances, and it is an object of the present invention to providean electrode catalyst that can exhibit sufficient catalytic performanceeven when it contains a relatively high chlorine concentration as highas more than 100 ppm.

Also, it is another object of the present invention to provide anelectrode catalyst that it is suitable for mass production due to thefact that there is required no special and complicated process foreliminating chlorine, and is also suitable for reducing themanufacturing cost.

Furthermore, it is a further object of the present invention to providea composition for forming a gas diffusion electrode, a gas diffusionelectrode, a membrane-electrode assembly (MEA), and a fuel cell stackthat include the aforementioned electrode catalyst.

Means to Solve the Problem

The present inventors, as a result of having performed intensivestudies, found out that it is possible to produce an electrode catalystwhich still exhibits a satisfactory performance (a core-shell catalystto be described later), even when containing such a high concentrationof chlorine as high as more than 100 ppm, by reducing the concentrationof bromine (Br) species contained in the electrode catalyst as measuredby X-ray fluorescence (XRF), and have completed the present invention.

More specifically, the present invention comprises the followingtechnical matters:

That is, the present invention

-   -   (1) provides an electrode catalyst having a core-shell structure        comprising:    -   a support;    -   a core part formed on said support; and    -   a shell part formed to cover at least a part of a surface of        said core part,    -   wherein the concentration of bromine (Br) species is not higher        than 500 ppm when measured by X-ray fluorescence (XRF)        spectroscopy, and the concentration of chlorine (Cl) species is        not higher than 8,500 ppm when measured by X-ray fluorescence        (XRF) spectroscopy.

Even when the concentration of chlorine (Cl) species contained in thecatalyst is extremely high as 8,500 ppm, the electrode catalyst of thepresent invention can exhibit a sufficient catalytic activity as anelectrode catalyst by controlling the concentration of the bromine (Br)species to 500 ppm or less. Further, the electrode catalyst is suitablefor mass production in that it does not require a special and complexmanufacturing process of removing chlorine, and is thus suitable forreducing the manufacturing cost.

In the present invention, a bromine (Br) species, refers to a chemicalspecies containing bromine as a constituent element. Specifically, thechemical species containing bromine include bromine atom (Br), brominemolecule (Br₂), bromide ion (Br—), bromine radical (Br.), polyatomicbromine ion and a bromine compound (e.g. X—Br where X represents acounterion).

In the present invention, the chlorine (Cl) species refers to a chemicalspecies containing chlorine as a constituent element. Specifically, thechemical species containing chlorine include chlorine atom (CO, chlorinemolecule (Cl₂), chloride ion (Cl⁻), chlorine radical (Cl.), polyatomicchloride ion and a chlorine compound (e.g. X—Cl where X represents acounterion).

In the present invention, bromine (Br) species concentration andchlorine (Cl) species concentration are measured by X-ray fluorescence(XRF) spectrometry. A value of the bromine (Br) species contained in theelectrode catalyst that is measured by X-ray fluorescence (XRF)spectrometry is the concentration of bromine (Br) species. Likewise, Avalue of the chlorine (Cl) species contained in the electrode catalystthat is measured by X-ray fluorescence (XRF) spectrometry is theconcentration of chlorine (Cl) species.

Here, the bromine (Br) species concentration and chlorine (Cl) speciesconcentration are concentrations of the bromine atoms and chlorine atomsin terms of the bromine element and chlorine element that arerespectively contained in the electrode catalyst.

Further, the present invention provides

-   -   (2) the electrode catalyst as set forth in (1), in which the        concentration of chlorine (Cl) species is not lower than 900        ppm.

In this way, the effects of the present invention can be achieved morereliably.

Furthermore, the present invention provides

-   -   (3) the electrode catalyst as set forth in (1) or (2), in which        the shell part contains at least one metal selected from        platinum (Pt) and a platinum (Pt) alloy, and the core part        contains at least one metal selected from the group consisting        of palladium (Pd), a palladium (Pd) alloy, a platinum (Pt)        alloy, gold (Au), nickel (Ni) and a nickel (Ni) alloy.

In this way, the effects of the present invention can be achieved morereliably. Further, by employing the abovementioned structure, there canbe achieved a higher catalytic activity and a higher durability.

Furthermore, the present invention provides

-   -   (4) the electrode catalyst as set forth in (3), in which the        support contains an electrically conductive carbon, the shell        part contains platinum (Pt) and the core part contains palladium        (Pd).

In this way, the effects of the present invention can be achieved morereliably. Further, by employing the abovementioned structure, there canbe achieved a higher catalytic activity and a higher durability.Furthermore, by employing the abovementioned structure, the electrodecatalyst of the present invention, as compared to a conventionalelectrode catalyst having a structure where platinum is supported on acarbon support, is capable of reducing the amount of platinum containedand is thus capable of easily reducing a raw material cost.

Furthermore, the present invention provides

-   -   (5) the electrode catalyst as set forth in (1) or (2), in which        the shell part has:        -   a first shell part formed to cover at least a part of the            surface of the core part; and        -   a second shell part formed to cover at least a part of a            surface of the first shell part.

In this way, the effects of the present invention can be achieved morereliably. By employing the abovementioned structure, the electrodecatalyst of the present invention is capable of reducing the containedamount of a noble metal(s) such as platinum used in the core part, andis thus capable of easily reducing a raw material cost.

Furthermore, the present invention provides

-   -   (6) the electrode catalyst as set forth in (5), in which the        first shell part contains palladium (Pd), and the second shell        part contains platinum (Pt).

In this way, the effects of the present invention can be achieved morereliably. Further, by employing the abovementioned structure, there canbe achieved a higher catalytic activity and a higher durability.

Furthermore, the present invention provides

-   -   (7) a composition for forming a gas diffusion electrode,        containing the electrode catalyst as set forth in any one of (1)        to (6).

According to the gas diffusion electrode-forming composition of thepresent invention, it is possible to easily produce a gas diffusionelectrode with a high catalytic activity (polarization property) becauseit contains the electrode catalyst of the present invention.

Furthermore, the present invention provides

-   -   (8) a gas diffusion electrode containing the electrode catalyst        as set forth in any one of (1) to (6).

According to the gas diffusion electrode of the present invention, it ispossible to achieve a high catalytic activity (polarization property)because it contains the electrode catalyst of the present invention.

Furthermore, the present invention provides

-   -   (9) a membrane-electrode assembly (MEA) including the gas        diffusion electrode as set forth in (8).

According to the membrane-electrode assembly (MEA) of the presentinvention, it is possible to achieve a high battery property because itcontains the gas diffusion electrode of the present invention.

Furthermore, the present invention provides

-   -   (10) a fuel cell stack including the membrane-electrode assembly        (MEA) as set forth in (9).

According to the fuel cell stack of the present invention, it ispossible to achieve a high battery property because it contains themembrane-electrode assembly (MEA) of the present invention.

Effects of the Invention

According to the present invention, there can be provided an electrodecatalyst that can exhibit a sufficient catalytic performance even whencontaining a relatively high concentration of chlorine as high as morethan 100 ppm,

Also, according to the present invention, there can be provided anelectrode catalyst that is suitable for mass production due to notgetting through the particular, complicated process for removal ofchlorine and is also suitable for reduction of the manufacturing cost.

Further, according to the present invention, there can be provided acomposition for forming a gas diffusion electrode, a gas diffusionelectrode, a membrane-electrode assembly (MEA), and a fuel cell stackthat include the aforementioned electrode catalyst.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic sectional view showing a preferred embodiment ofthe electrode catalyst of the present invention (core-shell catalyst).

FIG. 2 is a schematic sectional view showing another preferredembodiment of the electrode catalyst of the present invention(core-shell catalyst).

FIG. 3 is a schematic sectional view showing another preferredembodiment of the electrode catalyst of the present invention(core-shell catalyst).

FIG. 4 is a schematic sectional view showing another preferredembodiment of the electrode catalyst of the present invention(core-shell catalyst).

FIG. 5 is a schematic diagram showing a preferred embodiment of a fuelcell stack of the present invention.

FIG. 6 is a schematic diagram showing a schematic configuration of arotating disk electrode measuring device equipped with a rotating discelectrode used in a working example.

MODE FOR CARRYING OUT THE INVENTION

Preferable embodiments of the present invention are described in detailhereunder with reference to the drawings when necessary.

<Electrode Catalyst>

FIG. 1 is a schematic cross-sectional view showing a preferableembodiment of an electrode catalyst (core-shell catalyst) of the presentinvention.

As shown in FIG. 1, an electrode catalyst 1 of the present inventionincludes a support 2; and catalyst particles 3 supported on the support2 and having a so-called “core-shell structure.” Each catalyst particle3 has a core part 4; and a shell part 5 covering at least a part of thesurface of the core part 4. The catalyst particles 3 thus have aso-called “core-shell structure” including the core part 4 and the shellpart 5 formed on the core part 4.

That is, the electrode catalyst 1 has the catalyst particles 3 supportedon the support 2, and the catalyst particles 3 have the structure wherethe core part 4 serves as a core (core portion), and the shell part 5 asa shell covers at least a part of the surface of the core part 4.

Further, the constituent element (chemical composition) of the core part4 and the constituent element (chemical composition) of the shell part 5differ from each other in composition.

There are no particular restrictions on the electrode catalyst 1 of thepresent invention except that the shell part 5 has to be formed on atleast a part of the surface of the core part 4 of each catalyst particle3.

For example, in terms of more reliably achieving the effects of thepresent invention, it is preferred that the electrode catalyst 1 be in astate where the whole range of the surface of the core part 4 issubstantially covered by the shell part 5, as shown in FIG. 1.

Further, the electrode catalyst 1 may also be in a state where a part ofthe surface of the core part 4 is covered by the shell part 5, and therest part of the surface of the core part 4 is thus partially exposed,provided that the effects of the present invention can be achieved.

That is, with regard to the electrode catalyst of the present invention,it is sufficient that the shell part be formed on at least a part of thesurface of the core part.

FIG. 2 is a schematic cross-sectional view showing an other preferableembodiment (electrode catalyst 1A) of the electrode catalyst (core-shellcatalyst) of the present invention.

As shown in FIG. 2, an electrode catalyst 1A of the present inventionhas catalyst particles 3 a each being composed of a core part 4; a shellpart 5 a covering a part of the surface of the core part 4; and a shellpart 5 b covering an other part of the surface of the core part 4.

With regard to the catalyst particles 3 a contained in the electrodecatalyst 1A shown in FIG. 2, there is a part of the core part 4 that isneither covered by the shell part 5 a nor covered by the shell part 5 b.This part of the core part 4 composes a core part-exposed surface 4 s.

That is, as shown in FIG. 2, the catalyst particles 3 a contained in theelectrode catalyst 1A may also be in a state where the surface of thecore part 4 is partially exposed (e.g. a state where 4 s as a part ofthe surface of the core part 4 shown in FIG. 2 is exposed).

In other words, as is the case with the electrode catalyst 1A shown inFIG. 2, the shell part 5 a may be partially formed on a part of thesurface of the core part 4, and the shell part 5 b may then be partiallyformed on an other part of the surface of the core part 4.

FIG. 3 is a schematic cross-sectional view showing an other preferableembodiment (electrode catalyst 1B) of the electrode catalyst (core-shellcatalyst) of the present invention.

As shown in FIG. 3, an electrode catalyst 1B of the present inventionhas catalyst particles 3 each being composed of a core part 4; and ashell part 5 substantially covering the whole range of the surface ofthe core part 4.

The shell part 5 may have a two-layered structure composed of a firstshell part 6 and a second shell part 7. That is, the catalyst particles3 have a so-called “core-shell structure” comprised of the core part 4;and the shell part 5 (first shell part 6 and second shell part 7) formedon the core part 4.

The electrode catalyst 1B has a structure where the catalyst particles 3are supported on the support 2; the core part 4 of each catalystparticle 3 serves as a core (core portion); and the whole range of thesurface of the core part 4 is substantially covered by the shell part 5composed of the first shell part 6 and the second shell part 7.

Here, the constituent element (chemical composition) of the core part 4,the constituent element (chemical composition) of the first shell part 6and the constituent element (chemical composition) of the second shellpart 7 differ from one another in composition.

Moreover, the shell part 5 included in the electrode catalyst 1B of thepresent invention may further include an other shell part in addition tothe first shell part 6 and the second shell part 7.

In terms of more reliably achieving the effects of the presentinvention, it is preferred that the electrode catalyst 1B be in a statewhere the whole range of the surface of the core part 4 is substantiallycovered by the shell part 5, as shown in FIG. 3.

FIG. 4 is a schematic cross-sectional view showing an other preferableembodiment (electrode catalyst 1C) of the electrode catalyst (core-shellcatalyst) of the present invention.

As shown in FIG. 4, an electrode catalyst 1C of the present inventionhas catalyst particles 3 a each being composed of a core part 4; a shellpart 5 a covering a part of the surface of the core part 4; and a shellpart 5 b covering an other part of the surface of the core part 4.

The shell part 5 a may have a two-layered structure composed of a firstshell part 6 a and a second shell part 7 a.

Further, the shell part 5 b may have a two-layered structure composed ofa first shell part 6 b and a second shell part 7 b.

That is, the catalyst particles 3 a have a so-called “core-shellstructure” comprised of the core part 4; the shell part 5 a (first shellpart 6 a and second shell part 7 a) formed on the core part 4; and theshell part 5 b (first shell part 6 b and second shell part 7 b) formedon the core part 4.

With regard to the shell part 5 b composing the catalyst particle 3 ashown in FIG. 4, there is a part of the first shell part 6 b that is notcovered by the second shell part 7 b. The part of the first shell part 6b that is not covered by the second shell part 7 b composes a firstshell part-exposed surface 6 s.

With regard to the shell part 5 a composing the catalyst particle 3shown in FIG. 4, it is preferred that the whole range of the first shellpart 6 a be substantially covered by the second shell part 7 a.

Further, as shown in FIG. 4 and with regard to the shell part 5 bcomposing each catalyst particle 3 a, also permissible is a state wherea part of the surface of the first shell part 6 b is covered, and thesurface of the first shell part 6 b is thus partially exposed (e.g. astate shown in FIG. 4 where the part 6 s of the surface of the firstshell part 6 b is exposed), provided that the effects of the presentinvention can be achieved.

Moreover, on the premise that the effects of the present invention canbe achieved, the electrode catalyst 1 may allow a “complex of the corepart 4 and shell part 5 with the whole range of the surface of the corepart 4 being substantially covered by the shell part 5” and a “complexof the core part 4 and shell part 5 with the surface of the core part 4being partially covered by the shell part 5” to coexist on the support 2in a mixed manner.

Specifically, the electrode catalyst of the present invention may be ina state where the electrode catalysts 1 and 1A shown in FIGS. 1 and 2and the electrode catalysts 1B and 1C shown in FIGS. 3 and 4 coexist ina mixed manner, provided the effects of the present invention can beachieved.

Further, the electrode catalyst of the present invention may allow theshell part 5 a and the shell part 5 b to coexist in a mixed manner withrespect to an identical core part 4, as shown in FIG. 4, provided thatthe effects of the present invention can be achieved.

Furthermore, on the premise that the effects of the present inventioncan be achieved, the electrode catalyst of the present invention mayallow only the shell part 5 a to exist with respect to an identical corepart 4 or only the shell part 5 b to exist with respect to an identicalcore part 4 (none of these states are shown in the drawings).

Furthermore, on the premise that the effects of the present inventioncan be achieved, the electrode catalyst 1 may also be in a state where“particles only comprised of the core parts 4 that are not covered bythe shell parts 5” are supported on the support 2, in addition to atleast one kind of the electrode catalysts 1, 1A, 1B and 1C (not shown).

Furthermore, on the premise that the effects of the present inventioncan be achieved, the electrode catalyst 1 may also be in a state where“particles only composed of the constituent element of the shell part 5”are supported on the support 2 without being in contact with the coreparts 4, in addition to at least one kind of the electrode catalysts 1,1A, 1B and 1C (not shown).

Furthermore, on the premise that the effects of the present inventioncan be achieved, the electrode catalyst 1 may also be in a state where“particles only comprised of the core parts 4 that are not covered bythe shell parts 5” and “particles only composed of the constituentelement of the shell part 5” are individually and independentlysupported on the support 2, in addition to at least one kind of theelectrode catalysts 1, 1A, 1B and 1C.

It is preferred that the core part 4 have an average particle diameterof 2 to 40 nm, more preferably 4 to 20 nm, particularly preferably 5 to15 nm.

As for the thickness of the shell part 5 (thickness from the surface incontact with the core part 4 to the outer surface of the shell part 5),a preferable range thereof is to be appropriately determined based onthe design concept(s) of the electrode catalyst.

For example, when the amount of the metal element (e.g. platinum) usedto compose the shell part 5 is intended to be minimized, a layercomposed of one atom (one atomic layer) is preferred. In this case, whenthere is only one kind of metal element composing the shell part 5, itis preferred that the thickness of the shell part 5 be twice as large asthe diameter of one atom of such metal element (in sphericalapproximation). Further, when there are not fewer than two kinds ofmetal elements composing the shell part 5, it is preferred that thethickness of the shell part 5 be that of a layer of one atom (one atomiclayer formed with two or more kinds of atoms being apposed on thesurface of the core part 4).

Further, for example, when attempting to improve a durability byemploying a shell part 5 of a larger thickness, it is preferred thatsuch thickness be 1 to 10 nm, more preferably 2 to 5 nm.

When the shell part 5 has the two-layered structure composed of thefirst shell part 6 and the second shell part 7, preferable ranges of thethicknesses of the first shell part 6 and second shell part 7 areappropriately determined based on the design concept(s) of the electrodecatalyst of the present invention.

For example, when the amount of a noble metal such as platinum (Pt) as ametal element contained in the second shell part 7 is intended to beminimized, it is preferred that the second shell part 7 be a layercomposed of one atom (one atomic layer). In this case, when there isonly one kind of metal element composing the second shell part 7, it ispreferred that the thickness of the second shell part 7 be approximatelytwice as large as the diameter of one atom of such metal element(provided that an atom is considered as a sphere).

Further, when there are not fewer than two kinds of metal elementscontained in the second shell part 7, it is preferred that the secondshell part 7 have a thickness equivalent to that of a layer composed ofnot fewer than one kind of atom (one atomic layer formed with two ormore kinds of atoms being apposed in the surface direction of the corepart 4). For example, when attempting to improve the durability of theelectrode catalyst by employing a second shell part 7 of a largerthickness, it is preferred that the thickness of the second shell part 7be 1.0 to 5.0 nm. If the durability of the electrode catalyst is to befurther improved, it is preferred that the thickness of the second shellpart 7 be 2.0 to 10.0 nm.

Here, in the present invention, “average particle diameter” refers to anaverage value of the diameters of an arbitrary number of particles asparticle groups that are observed through electron micrographs.

There are no particular restrictions on the support 2, as long as suchsupport 2 is capable of supporting the catalyst particles 3 as thecomplexes composed of the core parts 4 and the shell parts 5, and has alarge surface area.

Moreover, it is preferred that the support 2 be that exhibiting afavorable dispersibility and a superior electrical conductivity in acomposition used to form a gas diffusion electrode having the electrodecatalyst 1.

The support 2 may be appropriately selected from carbon-based materialssuch as glassy carbon (GC), fine carbon, carbon black, black lead,carbon fiber, activated carbon, ground product of activated carbon,carbon nanofiber and carbon nanotube; and glass-based or ceramic-basedmaterials such as oxides.

Among these materials, carbon-based materials are preferred in terms oftheir adsorptivities with respect to the core part 4 and in terms of aBET specific surface area of the support 2.

Further, as a carbon-based material, an electrically conductive carbonis preferred. Particularly, an electrically conductive carbon black ispreferred as an electrically conductive carbon. Examples of suchelectrically conductive carbon black include products by the names of“Ketjenblack EC300 J,” “Ketjenblack EC600” and “Carbon EPC” (produced byLion Corporation).

There are no particular restrictions on the component of the core part4, as long as the component is capable of being covered by the shellpart 5.

When the shell part 5 employs a one-layered structure as are the caseswith the electrode catalysts 1 and 1A that are shown in FIGS. 1 and 2instead of the two-layered structure, the core part 4 may also employ anoble metal(s). The core part 4 composing the catalyst particles 3 and 3a of the electrode catalysts 1 and 1A, contains at least one metalselected from the group consisting of palladium (Pd), a palladium (Pd)alloy, a platinum (Pt) alloy, gold (Au), nickel (Ni) and a nickel (Ni)alloy.

There are no particular restrictions on a palladium (Pd) alloy, as longas the alloy is to be obtained by combining palladium (Pd) with an othermetal capable of forming an alloy when combined with palladium (Pd). Forexample, such palladium (Pd) alloy may be a two-component palladium (Pd)alloy obtained by combining palladium (Pd) with an other metal; or athree or more-component palladium (Pd) alloy obtained by combiningpalladium (Pd) with not fewer than two kinds of other metals.Specifically, examples of such two-component palladium (Pd) alloyinclude gold palladium (PdAu), silver palladium (PdAg) and copperpalladium (PdCu). One example of a three-component palladium (Pd) alloyis gold-silver-palladium (PdAuAg).

There are no particular restrictions on a platinum (Pt) alloy, as longas the alloy is to be obtained by combining platinum (Pt) with an othermetal capable of forming an alloy when combined with platinum (Pt). Forexample, such platinum (Pt) alloy may be a two-component platinum (Pt)alloy obtained by combining platinum (Pt) with an other metal; or athree or more-component platinum (Pt) alloy obtained by combiningplatinum (Pt) with not fewer than two kinds of other metals.Specifically, examples of such two-component platinum (Pt) alloy includenickel platinum (PtNi) and cobalt platinum (PtCo).

There are no particular restrictions on a nickel (Ni) alloy, as long asthe alloy is to be obtained by combining nickel (Ni) with an other metalcapable of forming an alloy when combined with nickel (Ni). For example,such nickel (Ni) alloy may be a two-component nickel (Ni) alloy obtainedby combining nickel (Ni) with an other metal; or a three ormore-component nickel (Ni) alloy obtained by combining nickel (Ni) withnot fewer than two kinds of other metals. Specifically, one example ofsuch two-component nickel (Ni) alloy is tungsten nickel (NiW).

The shell part 5 contains at least one kind of metal selected fromplatinum (Pt) and a platinum (Pt) alloy. There are no particularrestrictions on a platinum (Pt) alloy, as long as the alloy is to beobtained by combining platinum (Pt) with an other metal capable offorming an alloy when combined with platinum (Pt). For example, suchplatinum (Pt) alloy may be a two-component platinum (Pt) alloy obtainedby combining platinum (Pt) with an other metal; or a three ormore-component platinum (Pt) alloy obtained by combining platinum (Pt)with not fewer than two kinds of other metals. Specifically, examples ofsuch two-component platinum (Pt) alloy include nickel platinum (PtNi),cobalt platinum (PtCo), platinum ruthenium (PtRu), platinum molybdenum(PtMo) and platinum titanium (PtTi). Particularly, in order for theshell part 5 to have a poisoning resistance, it is preferred that aplatinum ruthenium (PtRu) alloy be used.

As are the cases with the electrode catalysts 1B and 1C that are shownin FIGS. 3 and 4, when the shell part 5 employs the two-layeredstructure composed of the first shell part 6 and the second shell part7, a metal element(s) other than noble metals may be the main componentespecially from the perspective of reducing the cost for producing theelectrode catalyst 1. Specifically, it is preferred that the core part 4be composed of a metal element(s) other than platinum (Pt) and palladium(Pd), a metal compound of such metal and/or a mixture of such metal andsuch metal compound. It is more preferred that the core part 4 becomposed of a metal element(s) other noble metals, a metal compound ofsuch metal and/or a mixture of such metal and such metal compound.

A supported amount of the platinum (Pt) contained in the shell part 5 is5 to 30% by weight, preferably 8 to 25% by weight with respect to theweight of the electrode catalyst 1. It is preferred that the amount ofthe platinum (Pt) supported be not smaller than 5% by weight, becausethe electrode catalyst can fully exert its catalytic activity in suchcase. It is also preferred that the amount of the platinum (Pt)supported be not larger than 30% by weight, because the amount ofplatinum (Pt) used is thus reduced in such case, which is favorable interms of production cost.

In the case where the shell part 5 has the two-layered structurecomposed of the first shell part 6 and the second shell part 7, it ispreferred that the first shell part 6 contain at least one kind of metalselected from the group consisting of palladium (Pd), a palladium (Pd)alloy, a platinum (Pt) alloy, gold (Au), nickel (Ni) and a nickel (Ni)alloy, and it is more preferred that the first shell part 6 containpalladium (Pd) simple substance.

From the perspective of further improving the catalytic activities ofthe electrode catalysts 1B and 1C and more easily obtaining the same, itis preferred that the first shell part 6 be mainly composed of palladium(Pd) simple substance (not less than 50 wt %), and it is more preferredthat such first shell part 6 be only composed of palladium (Pd) simplesubstance.

It is preferred that the second shell part 7 contain at least one kindof metal selected from platinum (Pt) and a platinum (Pt) alloy, and itis more preferred that such shell part 7 contain platinum (Pt) simplesubstance.

From the perspective of further improving the catalytic activities ofthe electrode catalysts 1B and 1C and more easily obtaining the same, itis preferred that the second shell part 7 be mainly composed of platinum(Pt) simple substance (not less than 50 wt %), and it is more preferredthat such second shell part 7 be only composed of platinum (Pt) simplesubstance.

(Concentration of Bromine (Br) Species and Concentration of Chlorine(Cl) Species)

The electrode catalyst 1 exhibits a bromine (Br) species concentrationof not higher than 500 ppm when measured through X-ray fluorescence(XRF) spectroscopy; and a chlorine (Cl) species concentration of nothigher than 8,500 ppm when measured through the same analytical method.

Even when the chlorine (Cl) species contained in the electrode catalyst1 is in an extremely high concentration of 8,500 ppm, the electrodecatalyst 1 is able to fully exert its catalytic activity by having abromine (Br) species concentration of not higher than 500 ppm. Further,the electrode catalyst 1 is suitable for mass production and productioncost reduction due to the fact that not special and complex productionprocess is required to remove chlorine.

Here, the bromine (Br) species concentration and the chlorine (Cl)species concentration are measured through X-ray fluorescence (XRF)spectroscopy. A value obtained by measuring the bromine (Br) speciescontained in the electrode catalyst through X-ray fluorescence (XRF)spectroscopy is the bromine (Br) species concentration. Similarly, avalue obtained by measuring the chlorine (Cl) species contained in theelectrode catalyst through X-ray fluorescence (XRF) spectroscopy is thechlorine (Cl) species concentration.

Here, the bromine (Br) species concentration and the chlorine (Cl)species concentration are respectively the concentrations of the bromineatoms and chlorine atoms in terms of the bromine and chlorine elementscontained in the electrode catalyst.

X-ray fluorescence (XRF) spectroscopy is a method where a specimencontaining a particular element A is irradiated with a primary X-ray togenerate a fluorescent X-ray of such element A, followed by measuringthe intensity of such fluorescent X-ray of the element A such thatquantitative analysis of the captioned element A contained in thespecimen can be performed. When performing quantitative analysis throughX-ray fluorescence (XRF) spectroscopy, there may be employed thefundamental parameter method (FP method) used in theoretical operation.

The FP method applies the idea that if the compositions and kinds of theelements contained in a specimen are all known, the fluorescent X-ray(XRF) intensities thereof can be individually and theoreticallycalculated. In addition, the FP method allows there to be estimated acomposition(s) corresponding to the fluorescent X-ray (XRF) of eachelement that is obtained by measuring the specimen.

X-ray fluorescence (XRF) spectroscopy is performed using generalfluorescent X-ray (XRF) analyzers such as an energy dispersivefluorescent X-ray (XRF) analyzer, a scanning-type fluorescent X-ray(XRF) analyzer and a multi-element simultaneous-type fluorescent X-ray(XRF) analyzer. A fluorescent X-ray (XRF) analyzer is equipped with asoftware which makes it possible to process the experimental dataregarding the correlation between the intensity of the fluorescent X-ray(XRF) of the element A and the concentration of the element A.

There are no particular restrictions on such software, as long as thesoftware is that generally used to perform X-ray fluorescence (XRF)spectroscopy.

For example, there may be employed a software for use in a generalfluorescent X-ray (XRF) analyzer adopting the FP method, such as ananalysis software: “UniQuant 5.” Here, one example of the abovementionedfluorescent X-ray (XRF) analyzer is a full-automatic wavelengthdispersive fluorescent X-ray analyzer (product name: Axios by SpectrisCo., Ltd.)

The electrode catalyst 1 exhibits a bromine (Br) species concentrationof not higher than 500 ppm when measured by the aforementioned X-rayfluorescence (XRF) spectroscopy. However, from the perspective offurther reliably achieving the effects of the present invention, itpreferred that the bromine (Br) species concentration be not higher than300 ppm, more preferably not higher than 200 ppm, and particularlypreferably not higher than 100 ppm. A bromine (Br) species concentrationof not higher than 500 ppm is preferable, because the electrode catalyst1 is capable of fully exerting its catalytic activity in such case evenwhen containing a chlorine (Cl) species of a high concentration.

In order to achieve a bromine (Br) species concentration of not higherthan 500 ppm when measured by the aforementioned X-ray fluorescence(XRF) spectroscopy, it is required that a metal compound as a staringmaterial of the electrode catalyst 1 and a reagent(s) used in eachproduction step of the electrode catalyst 1 be carefully selected.Specifically, there may, for example, be used a metal compound that doesnot generate bromine (Br) species, as the metal compound serving as thestarting material of the electrode catalyst 1. Further, there may, forexample, be employed a compound(s) that do not contain bromine (Br)species, as the reagent(s) used in the production steps of the electrodecatalyst 1.

Moreover, while the electrode catalyst 1 exhibits a chlorine (Cl)species concentration of not higher than 8,500 ppm when measured by theabovementioned X-ray fluorescence (XRF) spectroscopy, it is preferredthat such chlorine (Cl) species concentration be not higher than 7,500ppm, more preferably not higher than 6,500 ppm, even more preferably nothigher than 5,500 ppm, and particularly preferably not higher than 2,500ppm. In addition, it is especially preferred that the chlorine (Cl)species concentration be 1,000 ppm when measured by such X-rayfluorescence (XRF) spectroscopy.

It is preferable when the chlorine (Cl) species concentration is nothigher than 8,500 ppm, because the electrode catalyst 1 is capable offully exerting its catalytic activity under such condition due to thechlorine (Cl) species. Further, it is preferable when the chlorine (Cl)species concentration is not higher than 8,500 ppm, because theelectrode catalyst 1 can thus be produced without the production processof removing the chlorine (Cl) species, in the production process of theelectrode catalyst 1.

The electrode catalyst 1 of the present invention is capable of fullydelivering its performance as an electrode catalyst even when thechlorine (Cl) species concentration measured by the abovementioned X-rayfluorescence (XRF) spectroscopy is not lower than 900 ppm, or evengreater than 5,000 ppm.

That is, one technical feature of the electrode catalyst of the presentinvention is that bromine (Br) species is focused, and the bromine (Br)species concentration measured by the abovementioned X-ray fluorescence(XRF) spectroscopy is regulated to not higher than 500 ppm such that theelectrode catalyst is allowed to fully deliver its performance even whenthe chlorine (Cl) species concentration measured by the abovementionedX-ray fluorescence (XRF) spectroscopy is greater than 5,000 ppm (nothigher than 8,500 ppm).

In order to achieve a chlorine (Cl) species concentration of not higherthan 8,500 ppm when measured by the abovementioned X-ray fluorescence(XRF) spectroscopy, it is required that a metal compound as a staringmaterial of the electrode catalyst 1 and reagents used in productionsteps of the electrode catalyst be carefully selected. Specifically,there may, for example, be used a metal compound that does not generatechlorine (Cl) species, as the metal compound serving as the startingmaterial of the electrode catalyst 1. Further, there may, for example,be employed compounds that do not contain chlorine (Cl) species, as thereagents used in the production steps of the electrode catalyst 1.

Further, chlorine (Cl) species can be reduced to approximately severaltens of ppm by employing the chlorine reduction methods described later.

<Production Method of Electrode Catalyst>

A production method of the electrode catalyst 1 includes a step ofproducing an electrode catalyst precursor; and a step of washing suchcatalyst precursor to meet the condition where the bromine (Br) speciesconcentration measured by the X-ray fluorescence (XRF) spectroscopy isnot higher than 500 ppm, and the chlorine (Cl) species concentrationmeasured by the same method is not higher than 8,500 ppm.

(Production Step of Electrode Catalyst Precursor)

The electrode catalyst precursor of the electrode catalyst 1 is producedby having the support 2 support the catalyst components (core part 4,shell part 5) of the electrode catalyst.

There are no particular restrictions on a production method of theelectrode catalyst precursor as long as the method allows the catalystcomponents of the electrode catalyst 1 to be supported on the support 2.

Examples of the production method of the electrode catalyst precursorinclude an impregnation method where a solution containing the catalystcomponents of the electrode catalyst 1 is brought into contact with thesupport 2 to impregnate the support 2 with the catalyst components; aliquid phase reduction method where a reductant is put into a solutioncontaining the catalyst components of the electrode catalyst 1; anelectrochemical deposition method such as under-potential deposition(UPD); a chemical reduction method; a reductive deposition method usingadsorption hydrogen; a surface leaching method of alloy catalyst;immersion plating; a displacement plating method; a sputtering method;and a vacuum evaporation method.

(Concentration of Bromine (Br) Species and Concentration of Chlorine(Cl) Species)

Next, the concentrations of the bromine (Br) species and chlorine (Cl)species of the electrode catalyst precursor are adjusted to meet thecondition where the bromine (Br) species concentration measured by theX-ray fluorescence (XRF) spectroscopy is not higher than 500 ppm, andthe chlorine (Cl) species concentration measured by the same method isnot higher than 8,500 ppm. Specifically, there are employed thefollowing chlorine reduction methods 1 to 3.

[Chlorine Reduction Method 1]

A chlorine reduction method 1 includes a first step and a second step.

First step: The first step is to prepare a first liquid with anelectrode catalyst precursor (I) being dispersed in an ultrapure water.The first liquid is prepared by adding such electrode catalyst precursor(I) to the ultrapure water. Here, the electrode catalyst precursor (I)is produced using a material containing chlorine (Cl) species, andexhibits a chlorine (Cl) species concentration higher than apredetermined chlorine (Cl) species concentration when measured by theX-ray fluorescence (XRF) spectroscopy (e.g. an electrode catalystprecursor exhibiting a chlorine (Cl) species concentration value higherthan 8,500 ppm or 7,600 ppm, provided that 8,500 ppm or 7,600 ppm is thepredetermined chlorine (Cl) species concentration).

Second step: The second step is to prepare a second liquid with anelectrode catalyst precursor (II) being dispersed in the ultrapurewater. Specifically, the electrode catalyst precursor (I) contained inthe first liquid is filtrated and washed using the ultrapure water,followed by repeatedly washing the same until a filtrate obtained afterwashing has exhibited an electric conductivity ρ that is not higher thana predetermined value when measured by a JIS-standard testing method(JIS K0552) (e.g. not higher than a value predetermined within a rangeof 10 to 100 μS/cm). In this way, there is obtained the electrodecatalyst precursor (II) as well as the second liquid with such electrodecatalyst precursor (II) being dispersed in the ultrapure water.

[Chlorine Reduction Method 2]

A chlorine reduction method 2 includes a first step, a second step, athird step and a fourth step.

First step: The first step is to retain a liquid containing an ultrapurewater, a reductant and an electrode catalyst precursor under at leastone temperature predetermined within a range of 20 to 90° C. for apredetermined retention time. Here, the electrode catalyst precursor isproduced using a material containing chlorine (Cl) species, and exhibitsa chlorine (Cl) species concentration higher than a predeterminedchlorine (Cl) species concentration when measured by the X-rayfluorescence (XRF) spectroscopy (e.g. an electrode catalyst precursorexhibiting a chlorine (Cl) species concentration value higher than 8,500ppm or 6,000 ppm, provided that 8,500 ppm or 6,000 ppm is thepredetermined chlorine concentration).

Second step: The second step is to add the ultrapure water to the liquidobtained in the first step so as to prepare a first liquid where anelectrode catalyst precursor (I) contained in the liquid obtained in thefirst step is dispersed in the ultrapure water.

Third step: The third step is to filtrate and wash the electrodecatalyst precursor contained in the first liquid using the ultrapurewater, followed by repeatedly washing the same until a filtrate obtainedafter washing has exhibited an electric conductivity ρ that is nothigher than a predetermined first value when measured by a JIS-standardtesting method (JIS K0552). In this way, there is now obtained a secondliquid where dispersed in the ultrapure water is the electrode catalystprecursor contained in the liquid having an electric conductivity ρ thatis not higher than the predetermined first value.

Fourth step: The fourth step is to dry the second liquid.

[Chlorine Reduction Method 3]

A chlorine reduction method 3 includes a first step.

First step: The first step is to retain a liquid containing an ultrapurewater, a gas having hydrogen and an electrode catalyst precursor underat least one temperature predetermined within a range of 20 to 40° C.for a predetermined retention time. Here, the electrode catalystprecursor is produced using a material containing chlorine (Cl) species,and exhibits a chlorine (Cl) species concentration higher than apredetermined chlorine (Cl) species concentration when measured by theX-ray fluorescence (XRF) spectroscopy.

The “ultrapure water” used in the chlorine reduction methods 1 to 3 is atype of water exhibiting a specific resistance R of not lower than 3.0MΩ·cm, such specific resistance R being represented by the followinggeneral formula (1) (i.e. an inverse number of the electric conductivitymeasured by the JIS-standard testing method (JIS K0552)). Further, it ispreferred that the “ultrapure water” have a water quality equivalent toor clearer than “A3” as defined in JISK 0557 “Water used for industrialwater and wastewater analysis.”

[Formula 1]

R=1/ρ  (1)

In the above general formula (1), R represents the specific resistance,and ρ represents the electric conductivity measured by the JIS-standardtesting method (JIS K0552).

There are no particular restrictions on the ultrapure water, as long asthe water has an electric conductivity that satisfies the relationshiprepresented by the general formula (1). Examples of such ultrapure waterinclude an ultrapure water produced using an ultrapure water system from“Milli-Q series” (by Merck Ltd.); and an ultrapure water produced usingan ultrapure water system from “Elix UV series” (by Nihon MilliporeK.K.).

The chlorine (Cl) species contained in the electrode catalyst precursorcan be reduced by performing any one of the chlorine reduction methods 1to 3. Further, an electrode catalyst precursor exhibiting a bromine (Br)species concentration of not higher than 500 ppm and a chlorine (Cl)species concentration of not higher than 8,500 ppm when measured by theX-ray fluorescence (XRF) spectroscopy, is considered as the electrodecatalyst of the present invention.

The electrode catalyst is capable of exerting a level of catalyticactivity required as an electrode catalyst, due to the fact that theelectrode catalyst has a chlorine (Cl) species concentration of nothigher than 8,500 ppm and a bromine (Br) species concentration of nothigher than 500 ppm when measured by the X-ray fluorescence (XRF)spectroscopy.

(X-Ray Fluorescence (XRF) Spectroscopy)

The X-ray fluorescence (XRF) spectroscopy is, for example, performed inthe following manner.

(1) Measurement Device

-   -   Full-automatic wavelength dispersive fluorescent X-ray analyzer        Axios (by Spectris Co., Ltd.)

(2) Measurement Condition

-   -   Analysis software: “UniQuant 5” (Semi-quantitative analysis        software employing FP (four peak method))    -   XRF measurement chamber atmosphere: Helium (normal pressure)

(3) Measurement Procedure

-   Placing a sample-containing sample container into an XRF sample    chamber-   (ii) Replacing an atmosphere in the XRF sample chamber with helium    gas-   (iii) Setting the measurement condition to “UQ5 application” as a    condition required to use the analysis software “UniQuant 5” and    configuring a mode where calculation is performed in a mode with the    main component of the sample being “carbon (constituent element of    support)” and with a sample analysis result-display format being    “element,” under a helium gas atmosphere (normal pressure)

<Structure of Fuel Cell Stack>

FIG. 5 is a schematic view showing preferable embodiments of acomposition for forming gas diffusion electrode containing the electrodecatalyst of the present invention; a gas diffusion electrode producedusing such composition for forming gas diffusion electrode; amembrane-electrode assembly (MEA) having such gas diffusion electrode;and a fuel cell stack having such membrane-electrode assembly (MEA).

As for a fuel cell stack S shown in FIG. 5, each membrane-electrodeassembly (MEA) 400 serves as a one-unit cell, and the fuel cell stack Sis configured by stacking multiple layers of such one-unit cells.

Particularly, the fuel cell stack S has a membrane-electrode assembly(MEA) 400 that is equipped with an anode 200 a, a cathode 200 b and anelectrolyte membrane 300 provided between these electrodes.

More particularly, the fuel cell stack S has a structure where themembrane-electrode assembly (MEA) 400 is sandwiched between a separator100 a and a separator 100 b.

Described hereunder are the composition for forming gas diffusionelectrode, a gas diffusion electrode 200 a, a gas diffusion electrode200 b and the membrane-electrode assembly (MEA) 400, all of which serveas members of the fuel cell stack S containing the electrode catalyst ofthe present invention.

<Composition for Forming Gas Diffusion Electrode>

The electrode catalyst 1 can be used as a so-called catalyst inkcomponent and serve as the composition for forming gas diffusionelectrode in the present invention. One feature of the composition forforming gas diffusion electrode in the present invention is that thiscomposition contains the aforementioned electrode catalyst. The maincomponents of the composition for forming gas diffusion electrode arethe abovementioned electrode catalyst and an ionomer solution. Theionomer solution contains water, an alcohol and a polyelectrolyteexhibiting a hydrogen ion conductivity.

A mixing ratio between water and an alcohol in the ionomer solution canbe any ratio, as long as it is the kind of ratio capable of endowing aviscosity suitable for applying to the electrode the composition forforming gas diffusion electrode. In general, it is preferred that analcohol be contained in an amount of 0.1 to 50.0 parts by weight withrespect to 100 parts by weight of water. Further, it is preferred thatthe alcohol contained in the ionomer solution be a monohydric alcohol ora polyhydric alcohol. Examples of a monohydric alcohol include methanol,ethanol, propanol and butanol. Examples of a polyhydric alcohol includedihydric alcohols or trihydric alcohols. As a dihydric alcohol, therecan be listed, for example, ethylene glycol, diethylene glycol,tetraethylene glycol, propylene glycol, 1,3-butanediol and1,4-butanediol. As a trihydric alcohol, there may be used glycerin, forexample. Further, the alcohol contained in the ionomer solution may beeither one kind of alcohol or a combination of two or more kinds ofalcohols. Here, the ionomer solution may also be appropriately allowedto contain an additive(s) such as a surfactant, if necessary.

For the purpose of dispersing the electrode catalyst, the ionomersolution contains a hydrogen ion-conductive polyelectrolyte as a bindercomponent for improving an adhesion to a gas diffusion layer as a partcomposing the gas diffusion electrode. Although there are no particularrestrictions on the polyelectrolyte, examples of such polyelectrolyteinclude known perfluorocarbon resins having sulfonate groups and/orcarboxylic acid groups. As an easily obtainable hydrogen ion-conductivepolyelectrolyte, there can be listed, for example, Nafion (registeredtrademark of Du Pont), ACIPLEX (registered trademark of Asahi KaseiChemical Corporation) and Flemion (registered trademark of ASAHI GLASSCo., Ltd).

The composition for forming gas diffusion electrode can be produced bymixing, crushing and stirring the electrode catalyst and the ionomersolution. The composition for forming gas diffusion electrode may beprepared using crushing and mixing machines such as a ball mill and/oran ultrasonic disperser. A crushing and a stirring conditions at thetime of operating a crushing and mixing machine can be appropriatelydetermined in accordance with the mode of the composition for forminggas diffusion electrode.

It is required that the composition of each of the electrode catalyst,water, alcohol(s) and hydrogen ion-conductive polyelectrolyte that arecontained in the composition for forming gas diffusion electrode be thatcapable of achieving a favorable dispersion state of the electrodecatalyst, allowing the electrode catalyst to be distributed throughoutan entire catalyst layer of the gas diffusion electrode and improvingthe power generation performance of the fuel cell.

Particularly, it is preferred that the polyelectrolyte, alcohol(s) andwater be respectively contained in an amount of 0.1 to 2.0 parts byweight, an amount of 0.01 to 2.0 parts by weight and an amount of 2.0 to20.0 parts by weight with respect to 1.0 parts by weight of theelectrode catalyst. It is more preferred that the polyelectrolyte,alcohol(s) and water be respectively contained in an amount of 0.3 to1.0 parts by weight, an amount of 0.1 to 2.0 parts by weight and anamount of 5.0 to 6.0 parts by weight with respect to 1.0 parts by weightof the electrode catalyst. It is preferred that the composition of eachcomponent be within the abovementioned ranges, because when thecomposition of each component is within these ranges, not only a coatingfilm made of the composition for forming gas diffusion electrode willnot be spread extremely extensively on the gas diffusion electrode atthe time of forming the film, but the coating film formed of thecomposition for forming gas diffusion electrode is also allowed to havean appropriate and uniform thickness.

Here, the weight of the polyelectrolyte refers to a weight when it isdry i.e. a weight without a solvent in a polyelectrolyte solution,whereas the weight of water refers to a weight including a watercontained in the polyelectrolyte solution.

<Gas Diffusion Electrode>

The gas diffusion electrode (200 a, 200 b) of the present invention hasa gas diffusion layer 220; and an electrode catalyst layer 240 laminatedon at least one surface of the gas diffusion layer 220. Theaforementioned electrode catalyst is contained in the electrode catalystlayer 240 equipped to the gas diffusion electrode (200 a, 200 b). Thegas diffusion electrode 200 of the present invention can be used as ananode and an cathode.

In FIG. 5, the gas diffusion electrode 200 on the upper side is referredto as the anode 200 a, whereas the gas diffusion electrode 200 on thelower side is referred to as the cathode 200 b for the sake ofconvenience.

(Electrode Catalyst Layer)

In the case of the anode 200 a, the electrode catalyst layer 240 servesas a layer where a chemical reaction of dissociating a hydrogen gas sentfrom the gas diffusion layer 220 into hydrogen ions takes place due tothe function of the electrode catalyst 1 contained in the electrodecatalyst layer 240. Further, in the case of the cathode 200 b, theelectrode catalyst layer 240 serves as a layer where a chemical reactionof bonding an air (oxygen gas) sent from the gas diffusion layer 220 andthe hydrogen ions that have traveled from the anode through theelectrolyte membrane takes place due to the function of the electrodecatalyst 1 contained in the electrode catalyst layer 240.

The electrode catalyst layer 240 is formed using the abovementionedcomposition for forming gas diffusion electrode. It is preferred thatthe electrode catalyst layer 240 have a large surface area such that thereaction between the electrode catalyst 1 and the hydrogen gas or air(oxygen gas) sent from the diffusion layer 220 is allowed take place tothe fullest extent. Moreover, it is preferred that the electrodecatalyst layer 240 be formed in a manner such that the electrodecatalyst layer 240 has a uniform thickness as a whole. Although thethickness of the electrode catalyst layer 240 can be appropriatelyadjusted and there are no restrictions on such thickness, it ispreferred that the electrode catalyst layer 240 have a thickness of 2 to200 μm.

(Gas Diffusion Layer)

The gas diffusion layer 220 equipped to the gas diffusion electrode 200serves as a layer provided to diffuse to each of the correspondingelectrode catalyst layers 240 the hydrogen gas introduced from outsidethe fuel cell stack S into gas flow passages that are formed between theseparator 100 a and the gas diffusion layer 220 a; and the air (oxygengas) introduced from outside the fuel cell stack S into gas passagesthat are formed between the separator 100 b and the gas diffusion layer220 b. In addition, the gas diffusion layer 220 plays a role ofsupporting the electrode catalyst layer 240 to the gas diffusionelectrode 200 so as to immobilize the electrode catalyst layer 240 tothe surface of the gas diffusion electrode 220. The gas diffusion layer220 also plays a role of improving the contact between the electrodecatalyst 1 contained in the electrode catalyst layer 240 and thehydrogen gas as well as air (oxygen gas).

The gas diffusion layer 220 has a function of favorably passing thehydrogen gas or air (oxygen gas) supplied from the gas diffusion layer220 and then allowing such hydrogen gas or air to arrive at theelectrode catalyst layer 240. For this reason, it is preferred that thegas diffusion layer 220 have a water-repellent property such that a porestructure as a microstructure in the gas diffusion layer 220 will not beblocked by the electrode catalyst 1 and a water generated at the cathode200 b. Therefore, the gas diffusion layer 220 has a water repellentcomponent such as polyethylene terephthalate (PTFE).

There are no particular restrictions on a material(s) that can be usedin the gas diffusion layer 220. That is, there can be employed amaterial(s) known to be used in a gas diffusion layer of a fuel cellelectrode. For example, there may be used a carbon paper; or a materialmade of a carbon paper as a main raw material and an auxiliary rawmaterial applied to the carbon paper as the main raw material, suchauxiliary raw material being composed of a carbon powder as an optionalingredient, an ion-exchange water also as an optional ingredient and apolyethylene terephthalate dispersion as a binder. The thickness of thegas diffusion layer can be appropriately determined based on, forexample, the size of a cell used in a fuel cell. While there are noparticular restrictions on the thickness of the gas diffusion layer, athin gas diffusion layer is preferred for the purpose of ensuring ashort diffusion distance of a reactant gas. Meanwhile, since it isrequired that the gas diffusion layer also exhibit a mechanical strengthat the time of performing coating and during an assembly process, thereis usually used a gas diffusion layer having a thickness of about 50 to300 μm, for example.

As for the gas diffusion electrodes 200 a and 200 b, an intermediatelayer (not shown) may be provided between the gas diffusion layer 220and the electrode catalyst layer 240. In such case, each of the gasdiffusion electrodes 200 a and 200 b has a three-layered structurecomposed of the gas diffusion layer, the intermediate layer and thecatalyst layer.

(Production Method of Gas Diffusion Electrode)

A production method of the gas diffusion electrode is describedhereunder.

The production method of the gas diffusion electrode includes a step ofapplying to the gas diffusion layer 220 the composition for forming gasdiffusion electrode; and a step of forming the electrode catalyst layer240 by drying such gas diffusion layer 220 to which the composition forforming gas diffusion electrode has been applied. Specifically, thecomposition for forming gas diffusion electrode contains the ionomersolution composed of the electrode catalyst 1 with the catalystcomponents supported on the support; a hydrogen ion-conductivepolyelectrolyte; a water; and an alcohol(s).

The important point when applying to the gas diffusion layer 220 thecomposition for forming gas diffusion electrode is that the compositionfor forming gas diffusion electrode is to be homogeneously applied tothe gas diffusion layer 220. As a result of homogeneously applying thecomposition for forming gas diffusion electrode, there can be formed onthe gas diffusion layer 220 a coating film that has a uniform thicknessand is made of the composition for forming gas diffusion electrode.Although an application quantity of the composition for forming gasdiffusion electrode can be appropriately determined based on a mode ofusage of the fuel cell, it is preferred that the quantity be 0.1 to 0.5(mg/cm²) in terms of the amount of an active metal such as platinumcontained in the electrode catalyst layer 240, from the perspective of acell performance of a fuel cell having a gas diffusion electrode.

Next, after applying to the gas diffusion layer 220 the composition forforming gas diffusion electrode, the coating film of the composition forforming gas diffusion electrode that has been applied to the gasdiffusion layer 220 is dried so as to form the electrode catalyst layer240 on the gas diffusion layer 220. By heating the gas diffusion layer220 on which the coating film of the composition for forming gasdiffusion electrode has been formed, the water and alcohol(s) in theionomer solution contained in the composition for forming gas diffusionelectrode will be evaporated and thus disappear from the composition forforming gas diffusion electrode. As a result, in the step of applyingthe composition for forming gas diffusion electrode, the coating film ofthe composition for forming gas diffusion electrode that is formed onthe gas diffusion layer 220 becomes the electrode catalyst layer 240containing the electrode catalyst and polyelectrolyte.

<Membrane-Electrode Assembly (MEA)>

The membrane-electrode assembly 400 of the present invention (MembraneElectrode Assembly, abbreviated as MEA hereunder) has the anode 200 aand cathode 200 b which serve as the gas diffusion electrodes 200 usingthe electrode catalyst 1; and the electrolyte 300 dividing theseelectrodes. The membrane-electrode assembly (MEA) 400 can be produced bystacking the anode 200 a, the electrolyte 300 and the cathode 200 b inan order of anode 200 a, electrolyte 300 and cathode 200 b, and thenpressure-bonding the same.

<Fuel Cell Stack>

As for the fuel cell stack S of the present invention, the one-unit cell(single cell) is established with the separator 100 a (anode side) beingattached to an outer side of the anode 200 a of the membrane-electrodeassembly (MEA) 400 obtained, and with the separator 100 b (cathode side)being attached to an outer side of the cathode 200 b of themembrane-electrode assembly (MEA) 400, respectively. Further, the fuelcell stack S is obtained by integrating such one-unit cells (singlecells). Furthermore, a fuel cell system is completed by attaching aperipheral device(s) to the fuel cell stack S and assembling the same.

WORKING EXAMPLE

The present invention is described in greater detail hereunder withreference to working examples. However, the present invention is notlimited to the following working examples.

Here, the inventors of the present invention confirmed that iodine (I)species was not detected from the catalysts of the working andcomparative examples, when employing the X-ray fluorescence (XRF)spectroscopy.

Further, unless otherwise noted in the description of each step of thefollowing production method, these steps were carried out under a roomtemperature and in the air.

<Production of Electrode Catalyst>

Working Example 1

The electrode catalyst of the present invention was produced through thefollowing process. The raw materials of the electrode catalyst that wereused in the working examples are as follows.

-   -   Carbon black powder: product name “Ketjen Black EC300” (by        Ketjen Black International Co.)    -   Sodium tetrachloropalladate (II)    -   Palladium nitrate    -   Potassium chloroplatinate

[Preparation of Palladium-Supported Carbon]

As a support of the electrode catalyst, there was used a carbon blackpowder which was dispersed in a water to prepare a dispersion liquid of5.0 g/L. An aqueous solution of sodium tetrachloropalladate (II)(concentration 20% by mass) of 5 mL was then delivered by drops into andmixed with such dispersion liquid. An aqueous solution of sodium formate(100 g/L) of 100 mL was further delivered by drops into a dispersionliquid thus obtained, followed by taking the insoluble componentsthrough filtering and then washing the taken insoluble components with apure water. After performing drying, there was then obtained a palladium(core)-supported carbon with palladium being supported on carbon black.

[Copper (Cu) Covering Palladium (Core)]

An aqueous solution of copper sulfate of 50 mM was poured into athree-electrode electrolytic cell. A reasonable amount of thepalladium-supported carbon prepared above was then added to suchthree-electrode electrolytic cell, followed by stirring the same andthen allowing the three-electrode electrolytic cell to stand still. 450mV (pair reversible hydrogen electrode) was applied to the workingelectrode in a resting state such that copper (Cu) could uniformly coatthe palladium of the palladium-supported carbon. This is defined as acopper-palladium supported carbon.

[Platinum (Pt) Covering Palladium (Core)]

An aqueous solution of potassium chloroplatinic acid was delivered bydrops into the solution containing the copper-palladium supported carbonwith palladium being coated by copper, the aqueous solution of potassiumchloroplatinic acid containing platinum (Pt) in an amount two-foldequivalent of the coating copper in terms of substance amount ratio. Inthis way, the copper (Cu) of the copper-palladium supported carbon wasreplaced with platinum (Pt).

[Washing and Drying]

After filtering a powder of the particles of such platinumpalladium-supported carbon obtained by replacing the copper (Cu) of thecopper-palladium supported carbon with platinum, without performingdrying, an ultrapure water was used to wash the same in a wet state dueto a filtrate, followed by drying the same at a temperature of 70° C.Thus, there was obtained an electrode catalyst of the working example 1which was {platinum (Pt)-palladium (Pd) supported carbon (core part:palladium, shell part: platinum)}.

[Measurement of Supported Amount]

With regard to the electrode catalyst of the working example 1, theamounts (% by mass) of the platinum and palladium supported weremeasured by the following method.

The electrode catalyst of the working example 1 was immersed in an aquaregia to dissolve the metal. Then, carbon as an insoluble component wasremoved from the aqua regia. Next, the aqua regia from which carbon hadbeen removed was subjected to ICP analysis.

The results of ICP analysis were that a platinum supporting amount was19.3% by mass, and a palladium supporting amount was 24.1% by mass.

Working Examples 2 to 15, Working Example 17

Except the fact that the supporting amounts of the platinum (Pt) andpalladium (Pd) contained in the electrode catalyst became thoserepresented by the concentrations listed in Tables 1 and 2 (% by massconcentration), electrode catalysts of working examples 2 to 15 and 17were produced in a similar manner as the working example 1.

Working Example 16

Except the fact that a palladium salt as a raw material of the electrodecatalyst was changed to achieve the supporting amounts of the platinum(Pt) and palladium (Pd) contained in the electrode catalyst as thoserepresented by the concentrations (% by mass concentration) in Table 1,an electrode catalyst of working example 16 was produced in a similarmanner as the working example 1.

Working Example 18

An electrode catalyst was prepared in a similar manner as the workingexample 1. This electrode catalyst was further soaked into an aqueoussolution of sulfuric acid (1M) at a normal temperature and for apredetermined period of time. Then, the electrode catalyst in theaqueous solution of sulfuric acid was filtered and washed with anultrapure water. Next, the electrode catalyst was immersed in an aqueoussolution of oxalic acid (0.3M) and retained at a temperature of 90° C.for a predetermined period of time. Next, the electrode catalyst in theaqueous solution of oxalic acid was filtered and washed with theultrapure water. Next, the electrode catalyst that had been washed withthe ultrapure water was dried at a temperature of 70° C. In this way, anelectrode catalyst of a working example 18 was obtained.

Further, ICP analysis was performed in a similar manner as the workingexample 1 for the purpose of measuring the supporting amounts ofplatinum and palladium.

Working Examples 19-20

An electrode catalyst was prepared in a similar manner as the workingexample 1. This electrode catalyst was further immersed in an aqueoussolution of sodium formic acid (0.01M) and retained at a normaltemperature and for a predetermined period of time. Next, the electrodecatalyst in the aqueous solution of sodium formic acid was filtered andwashed with an ultrapure water. Next, the electrode catalyst that hadbeen washed with the ultrapure water was dried at a temperature of 70°C. In this way, electrode catalysts of working examples 19 to 20 wereobtained.

Further, ICP analysis was performed in a similar manner as the workingexample 1 for the purpose of measuring the supporting amounts ofplatinum and palladium.

Working Example 21

An electrode catalyst was prepared in a similar manner as the workingexample 1. This electrode catalyst was immersed in an aqueous solutionof sodium formic acid (0.01M) and retained at a normal temperature andfor a predetermined period of time. Next, the electrode catalyst in theaqueous solution of sodium formic acid was filtered and washed with anultrapure water.

The filtered and washed electrode catalyst was further soaked in anaqueous solution of sulfuric acid (1M) at a normal temperature for apredetermined period of time. Next, the electrode catalyst in theaqueous solution of sulfuric acid was filtered and washed with theultrapure water. Next, the electrode catalyst was immersed in an aqueoussolution of oxalic acid (0.3M) and retained at 90° C. for apredetermined period of time. Next, the electrode catalyst in theaqueous solution of oxalic acid was filtered and washed with theultrapure water. Next, the electrode catalyst that had been washed withthe ultrapure water was dried at a temperature of 70° C. In this way, anelectrode catalyst of a working example 21 was obtained.

Further, ICP analysis was performed in a similar manner as the workingexample 1 for the purpose of measuring the supporting amounts ofplatinum and palladium.

Working Example 22

An electrode catalyst was prepared in a similar manner as the workingexample 1. This electrode catalyst was further immersed in an aqueoussolution of sodium formic acid (0.01M) and retained at 90° C. for apredetermined period of time. Next, the electrode catalyst in theaqueous solution of sodium formic acid was filtered and washed with anultrapure water. Next, the electrode catalyst that had been washed withthe ultrapure water was dried at a temperature of 70° C. In this way, anelectrode catalyst of a working example 22 was obtained.

Further, ICP analysis was performed in a similar manner as the workingexample 1 for the purpose of measuring the supporting amounts ofplatinum and palladium.

Working Example 23

An electrode catalyst was prepared in a similar manner as the workingexample 1. Then, the electrode catalyst was immersed in an aqueoussolution of oxalic acid (0.3M) and retained at 90° C. for apredetermined period of time. Next, the electrode catalyst in theaqueous solution of oxalic acid was filtered and washed with anultrapure water. Next, the electrode catalyst that had been washed withthe ultrapure water was dried at a temperature of 70° C. In this way, anelectrode catalyst of a working example 23 was obtained.

Further, ICP analysis was performed in a similar manner as the workingexample 1 for the purpose of measuring the supporting amounts ofplatinum and palladium.

Comparative Examples 1 to 7

Except the fact that the bromine species concentration was adjusted tothat shown in Table 3 by using, as a raw material, a potassiumchloroplatinic acid whose bromine concentration is 10,000 to 13,000 ppm,the electrode catalysts of comparative examples 1 to 7 were produced ina similar manner as the working example 1.

(Concentrations of Bromine (Br) Species and Chlorine (Cl) Species)

X-ray fluorescence (XRF) spectrometry was employed to measure theconcentrations of the bromine (Br) species and chlorine (Cl) species ofthe electrode catalysts that are obtained in the working examples 1 to23, and the comparative examples 1 to 7. The concentrations of thebromine species and chlorine species in the electrode catalysts weremeasured using the wavelength dispersive fluorescent X-ray analyzerAxios (by Spectris Co., Ltd.). Specifically, the measurement was carriedout through the following procedure.

A measurement sample of the electrode catalyst was placed in a XRFsample container equipped to the wavelength dispersive fluorescent X-rayanalyzer. The XRF sample container in which the measurement sample ofthe electrode catalyst had been placed was then put into an XRF samplechamber, followed by replacing an atmosphere in the XRF sample chamberwith a helium gas. Later, fluorescent X-ray measurement was conductedunder the helium gas atmosphere (normal pressure).

As a software, there was used “UniQuant5” which is an analytic softwarefor use in wavelength dispersive fluorescent X-ray analyzer. Ameasurement condition(s) were set to “UQ5 application” in accordancewith the analytic software “UniQuant5,” where calculation is performedin a mode with the main component of the measurement sample of theelectrode catalyst being “carbon (constituent element of electrodecatalyst support)” and with a sample analysis result-display formatbeing “element.” Measurement results were analyzed using the analyticsoftware “UniQuant5” such that the concentrations of bromine (Br)species and chlorine (Cl) species were able to be calculated. Themeasurement results are shown in Tables 1 to 3.

<Measurement of Evaluation (ik) of Catalytic Activity>

The catalytic activities of the electrode catalysts produced in theworking examples 1 to 23, and the comparative examples 1 to 7, wereevaluated by a rotating disk electrode method (RDE method). Thecatalytic activities of the electrode catalysts were measured by therotating disk electrode method (RDE method) in the following manner.

(Production of Composition for Forming Gas Diffusion Electrode)

A powder of each of the electrode catalysts produced in the workingexamples 1 to 23 and the comparative examples 1 to 7 was taken by anamount of about 8.0 mg through measurement, and was put into a samplebottle together with an ultrapure water of 2.5 mL, followed by mixingthe same while under the influence of an ultrasonic irradiation, thusproducing a slurry (suspension) of the electrode catalyst. Next, therewas prepared a Nafion-ultrapure water solution by mixing an ultrapurewater of 10.0 mL and a 10 wt % Nafion (registered trademark) dispersionaqueous solution (product name “DE1020CS” by Wako Chemical Ltd.) of 20μL in a different container. The Nafion-ultrapure water solution of 2.5mL was slowly poured into the sample bottle containing the slurry(suspension) of the electrode catalyst, followed by thoroughly stirringthe same at a room temperature for 15 min while under the influence ofan ultrasonic irradiation, thus obtaining a composition for forming gasdiffusion electrode.

(Electrode Catalyst Layer Formation)

FIG. 6 is a schematic diagram showing a schematic configuration of arotating disk electrode measuring device D used in the rotating diskelectrode method (RDE method).

As shown in FIG. 6, the rotating disk electrode measuring device Dmainly includes a measuring device cell 10, a reference electrode (RE)20, a counter electrode (CE) 30, a rotating disk electrode 40 and anelectrolyte solution 60.

An electrode catalyst layer X was formed on the surface of the rotatingdisk electrode 40 equipped to the rotating disk electrode measuringdevice D. Further, the catalyst of the electrode catalyst layer X wasevaluated by the rotating disk electrode method.

Particularly, there was used a rotating disk electrode measuring deviceD (model HSV110 by Hokuto Denko Corp.) employing HClO₄ of 0.1M as theelectrolyte 60, an Ag/AgCl saturated electrode as the referenceelectrode (RE) 20 and a Pt mesh with Pt black as the counter electrode(CE) 30.

A method for forming the electrode catalyst layer X on the surface ofthe rotating disk electrode 40 is described hereunder.

The composition for forming gas diffusion electrode that had beenproduced above was taken by an amount of 10 μL, and was delivered bydrops onto the surface of the clean rotating disk electrode (made ofglassy carbon, diameter 5.0 mmφ, area 19.6 mm²). Later, the compositionfor forming gas diffusion electrode was spread on the entire surface ofthe rotating disk electrode to form a uniform and given thickness,thereby forming on the surface of the rotating disk electrode a coatingfilm made of the composition for forming gas diffusion electrode. Thecoating film made of the composition for forming gas diffusion electrodewas dried under a temperature of 23° C. and a humidity of 50% RH for 2.5hours, thus forming the electrode catalyst layer X on the surface of therotating disk electrode 40.

(Measurement by Rotating Disk Electrode Method (RDE Method))

Measurements by the rotating disk electrode method include performingcleaning inside the rotating disk electrode measuring device; anevaluation of electrochemical surface area (ECSA) prior to themeasurement; an evaluation of electrochemical surface (ECSA) before andafter an oxygen reduction (ORR) current measurement.

[Cleaning]

In the rotating disk electrode measuring device D, after soaking therotating disk electrode 40 in HClO₄ electrolyte solution 60, theelectrolyte solution 60 was purged with an argon gas for not shorterthan 30 min. Then, potential scan was performed for 20 cycles under thecondition where the scanning potential was set to be 85˜1,085 mV vsRHE,and the scanning speed was set to be 50 my/sec.

[Evaluation of Electrochemical Surface Area (ECSA) Before Measurement]

Then, potential scan was performed for three cycles under the conditionwhere the scanning potential was set to be 50˜1,085 mV vsRHE, and thescanning speed was set to be 20 mV/sec.

[Oxygen Reduction (ORR) Current Measurement]

After purging the electrolyte solution 60 with an oxygen gas for notshorter than 15 minutes, a cyclic voltammogram (CV) measurement wasperformed for 10 cycles under the condition where the scanning potentialwas set to be 135 to 1,085 mV vsRHE, the scanning speed was set to be 10mV/sec, and the rotation speed of the rotating disk electrode 40 was setto be 1,600 rpm. An electrical current value at a potential of 900 mVvsRHE was recorded. In addition, the rotation speed of the rotating diskelectrode 40 was individually set to be 400 rpm, 625 rpm, 900 rpm, 1,225rpm, 2,025 rpm, 2,500 rpm and 3,025 rpm, and an oxygen reduction (ORR)current measurement was carried out per each cycle. A currentmeasurement value was defined as an oxygen reduction current value (i).

[Evaluation of Electrochemical Surface Area (ECSA after Measurement)]

Finally, the cyclic voltammogram (CV) measurement was performed forthree cycles under the condition where the scanning potential was set tobe 50 to 1,085 mV vsRHE, and the scanning speed was set to be 20 mV/sec.

(Calculation of Catalytic Activity)

The catalytic activity of the electrode catalyst was calculated using acorrection formula of mass transfer which is based on a Nernstdiffusion-layer model as shown by the following general formula (2),with the aid of the oxygen reduction current value (i) obtained aboveand a limiting current value (iL) measured in the cyclic voltammogram(CV) measurement. The calculation results of the working examples 1 to17 are shown in Table 1, and the calculation results of the workingexamples 18 to 23 are shown in Table 2. In addition, Table 3 shows thecalculation results of the comparative examples 1 to 7.

$\begin{matrix}\left\lbrack {{Formula}\mspace{14mu} 2} \right\rbrack & \; \\{{ik} = \frac{{iL} \times i}{{iL} - i}} & (2)\end{matrix}$

(In the general formula (2), i represents the oxygen reduction current(ORR current) measurement value, iL represents the limiting diffusioncurrent measurement value, ik represents the catalytic activity.)

TABLE 1 Bromine Chlorine Pt/% Pd/% species species Working by byconcentra- concentra- example mass mass tion/ppm tion/ppm ik/mA 1 19.324.1 200 6100 1.96 2 23.8 21.9 200 8400 1.90 3 20.5 24.4 100 7400 2.25 423.5 22.4 100 7500 2.64 5 23.7 22.0 100 7600 2.75 6 19.5 24.2 100 61002.41 7 23.5 21.5 300 7800 1.97 8 20.6 23.7 100 6000 2.32 9 20.4 23.2 1006100 1.91 10 24.3 21.1 100 8500 2.07 11 21.4 22.0 200 6100 2.51 12 22.922.5 100 5700 2.47 13 20.6 23.7 100 5200 2.45 14 22.9 21.8 100 6600 2.0315 23.7 22.0 300 6100 1.68 16 24.3 21.2 100 8500 2.11 17 19.5 24.2 1006000 2.32

TABLE 2 Bromine Chlorine Pt/% Pd/% species species Working by byconcentra- concentra- example mass mass tion/ppm tion/ppm ik/mA 18 21.023.0 100 2200 1.72 19 22.8 22.7 100 0 1.99 20 19.6 24.4 100 0 2.16 2120.0 23.5 100 0 2.13 22 21.0 22.9 500 0 1.74 23 23.5 21.5 100 900 2.20

TABLE 3 Bromine Chlorine Pt/% Pd/% species species Comparative by byconcentra- concentra- example mass mass tion/ppm tion/ppm ik/mA 1 20.922.5 3000 6900 1.40 2 18.5 24.4 4100 3900 1.25 3 21.3 22.8 6800 56001.65 4 21.9 22.9 6600 5200 1.46 5 22 22.4 8700 8900 1.20 6 21.9 22.87000 6100 1.64 7 20.9 22.5 3900 1900 1.40

According to Table 1 and Table 2, it can be understood that even whencontaining a chlorine (Cl) species of a high concentration, theelectrode catalyst containing a finely controlled amount of bromine (Br)species was able to exhibit a favorable catalytic activity.

Especially, each of the electrode catalysts shown in Table 1 exhibits achlorine (Cl) concentration greater than 5,000 ppm. However, since theseelectrode catalysts have their bromine (Br) species concentrationscontrolled to 100 to 300 ppm, favorable catalytic activities areexhibited.

Meanwhile, according to Table 3, it can be understood that the electrodecatalysts whose bromine (Br) species concentrations were greater than500 ppm exhibited decreased catalytic activities. That is, it becameclear that even when containing a chlorine (Cl) species of a highconcentration (e.g. greater than 5,000 ppm), an electrode catalystcontaining a finely controlled amount of bromine (Br) species is able toexhibit a significantly favorable catalytic activity, and is alsosuitable for mass production and reducing a production cost.

DESCRIPTION OF THE SYMBOLS

-   1 Electrode catalyst-   1A Electrode catalyst-   1B Electrode catalyst-   1C Electrode catalyst-   2 Support-   3 Catalyst particle-   3 a Catalyst particle-   4 Core part-   4 s Core part exposed surface-   5 Shell part-   6 First shell part-   6 s First shell part exposed surface-   7 Second shell part-   S Fuel cell stack-   100 Separator-   100 a Separator (anode side)-   100 b Separator (cathode side)-   200 Gas diffusion electrode-   200 a Gas diffusion electrode (anode)-   200 b Gas diffusion electrode (cathode)-   220 Gas diffusion layer-   240 Electrode catalyst layer-   300 Electrolyte-   400 Membrane-electrode assembly (MEA)-   X Electrode catalyst layer-   D Rotating disk electrode (RDE) measuring device-   10 Measuring device cell-   12 Gas introduction inlet-   20 Reference electrode (RE)-   22 Reference electrode (RE) cell-   30 Counter electrode (CE)-   40 Rotating disc electrode-   42 Electrode base material-   50 Solid table-   52 Supporting part-   54 Oil seal-   60 Electrolyte solution

INDUSTRIAL APPLICABILITY

The electrode catalyst of the present invention is a type of catalystcapable of demonstrating a sufficient catalytic performance even whenhaving a chlorine content of a high concentration. The catalystelectrode is also able to simplify a production process thereof, and isthus suitable for reducing a production cost and conducting massproduction. For these reasons, the present invention is a type ofelectrode catalyst that can be used not only in fuel-cell vehicles andelectrical equipment industries such as those related to cellularmobiles, but also in Ene farms, cogeneration systems or the like. Thus,the electrode catalyst of the present invention shall make contributionsto the energy industries and developments related to environmentaltechnologies.

1. An electrode catalyst having a core-shell structure comprising: a support; a core part formed on said support; and a shell part formed to cover at least a part of a surface of said core part, wherein a concentration of bromine (Br) species is not higher than 500 ppm when measured by X-ray fluorescence (XRF) spectroscopy, and a concentration of chlorine (Cl) species is in a range from 900 ppm to 8,500 ppm when measured by X-ray fluorescence (XRF) spectroscopy.
 2. (canceled)
 3. The electrode catalyst according to claim 1, wherein said shell part contains at least one metal selected from platinum (Pt) and a platinum (Pt) alloy, and said core part contains at least one metal selected from the group consisting of palladium (Pd), a palladium (Pd) alloy, a platinum (Pt) alloy, gold (Au), nickel (Ni) and a nickel (Ni) alloy.
 4. The electrode catalyst according to claim 3, wherein said support contains an electrically conductive carbon, said shell part contains platinum (Pt) and said core part contains palladium (Pd).
 5. The electrode catalyst according to claim 1, wherein said shell part has: a first shell part formed to cover at least a part of the surface of said core part; and a second shell part formed to cover at least a part of a surface of said first shell part.
 6. The electrode catalyst according to claim 5, wherein said first shell part contains palladium (Pd), and said second shell part contains platinum (Pt).
 7. A composition for forming a gas diffusion electrode, containing the electrode catalyst as set forth in claim
 1. 8. A gas diffusion electrode containing the electrode catalyst as set forth in claim
 1. 9. A membrane-electrode assembly (MEA) including the gas diffusion electrode as set forth in claim
 8. 10. A fuel cell stack including the membrane-electrode assembly (MEA) as set forth in claim
 9. 11. A composition for forming a gas diffusion electrode, containing the electrode catalyst as set forth in claim
 3. 12. A composition for forming a gas diffusion electrode, containing the electrode catalyst as set forth-in claim
 4. 13. A composition for forming a gas diffusion electrode, containing the electrode catalyst as set forth in claim
 5. 14. A composition for forming a gas diffusion electrode, containing the electrode catalyst as set forth in claim
 6. 15. A gas diffusion electrode containing the electrode catalyst as set forth in claim
 3. 16. A gas diffusion electrode containing the electrode catalyst as set forth in claim
 4. 17. A gas diffusion electrode containing the electrode catalyst as set forth in claim
 5. 18. A gas diffusion electrode containing the electrode catalyst as set forth in claim
 6. 19. A membrane-electrode assembly (MEA) including the gas diffusion electrode as set forth in claim
 15. 20. A membrane-electrode assembly (MEA) including the gas diffusion electrode as set forth in claim
 16. 21. A membrane-electrode assembly (MEA) including the gas diffusion electrode as set forth in claim
 17. 22. A membrane-electrode assembly (MEA) including the gas diffusion electrode as set forth in claim
 18. 23. A fuel cell stack including the membrane-electrode assembly (MEA) as set forth in claim
 19. 24. A fuel cell stack including the membrane-electrode assembly (MEA) as set forth in claim
 20. 25. A fuel cell stack including the membrane-electrode assembly (MEA) as set forth in claim
 21. 26. A fuel cell stack including the membrane-electrode assembly (MEA) as set forth in claim
 22. 